Use dyn.load or library.dynam to call a C ++ function in package R

I am very new to R. I would like to create an R package that will call a C ++ function using .Call (). I have a NAMESPACE file, with

useDynLib(mypkg)

where mypkg is also the function name of my C ++ code.

It works if I use this line at the beginning of mypkg.R:

dyn.load("src/mypkg.so")

but I want to use library.dynam instead, so in the zzz.R file I put

.onLoad<-function(libname, pkgname)
 {
 library.dynam("mypkg", pkgname, libname)
 }

It gives an error while checking the package:

...
Error in .Call("mypkg", PACKAGE = "mypkg") : 
C symbol name "mypkg" not in DLL for package "mypkg".
Error : unable to load R code in package 'mypkg'
...

It looks like the * .so file is being generated in the wrong place? Why is the / libs folder not created?

I would like the package to be os independent, is there any way to do this with dyn.load?

And this can be a very stupid question, where do pkgname and libname get their data from?

Many thanks for your help.